1. Field of the Invention
The present invention relates to a Wideband Code Division Multiple Access (herein after referred to as ‘WCDMA’) communication system. More particularly, the present invention relates to a method and an apparatus for selectively using an Enhanced Uplink Dedicated Channel (hereinafter referred to as ‘EUDCH’ or ‘E-DCH’) or an Uplink Dedicated Channel (hereinafter referred to as ‘DCH’) as a transmission path for uplink data transmission when a User Equipment (hereinafter referred to as ‘UE’) passes through a network which includes cells supporting the E-DCH and cells not supporting the E-DCH.
2. Description of the Related Art
Universal Mobile Telecommunication Service (hereinafter referred to as ‘UMTS’) is a 3rd generation mobile communication system based upon European mobile communication systems. That is, UMTS is a system based upon Global System for Mobile Communications (GSM) and General Packet Radio Services (GPRS) employing a WCDMA scheme. UMTS provides uniform services which enables cellular phone or computer users to transmit packet-based text, digitized voice or video and multimedia data at a speed higher than 2 Mbps no matter where in the world they are located.
FIG. 1 illustrates a structural view of a typical UMTS Terrestrial Radio Access Network (hereinafter referred to as ‘UTRAN’).
Referring to FIG. 1, the UTRAN 111 consisting of Radio Network Controllers (hereinafter referred to as ‘RNC’) 131, 132 and Node Bs 141, 142, 143, 144 connects a UE 151 to a Core Network (hereinafter referred to as ‘CN’) 101. Node Bs 141 to 144 may include a plurality of subordinate cells, and each RNC 131, 132 controls Node Bs under its control and each Node B also controls cells under its control. Each RNC 131, 132, Node Bs under the control of each RNC 131, 132, and the cells under the controls of the Node Bs constitute a Radio Network Subsystem (hereinafter referred to as ‘RNS’) 121, 122.
The RNCs 131, 132 assign or manage radio resources of the Node Bs 141 to 144 under their control, and the Node Bs 141 to 144 functions to actually provide the radio resources. The radio resources are configured cell by cell, and the radio resources provided by the Node Bs 141 to 144 signify the radio resources of the cells under their control. The UE 151 configures a radio channel and carries out communication using the radio resource which is provided by a specific cell of a specific Node B. As for the UE 151, the discrimination between the Node B and the cell is meaningless because it recognizes only physical channels configured cell by cell. Thus, the Node B and the cell will be henceforth referred to as terms having the same meaning.
An interface between the IE and the RNC is called a Uu interface, and FIG. 2 shows a layered architecture of the Uu interface in detail. The Uu interface is divided into a Control Plane (hereinafter referred to as ‘C-Plane’) 201 and a User Plane (hereinafter referred to as ‘U-Plane’) 202. The C-Plane 201 is used for exchanging control signals between the UE and the RNC, and the U-Plane 202 is used for actually transmitting user data.
A Radio Resource Control (hereinafter referred to as ‘RRC’) layer 211, a Radio Link Control (hereinafter referred to as ‘RLC) layer 241, a Media Access Control (hereinafter referred to as “MAC’) layer 271 and a physical layer 291 exist in the C-Plane 201, and a Packet Data Convergence Protocol (hereinafter referred to as ‘PDCP’) layer 221, a Broadcast Management Control (hereinafter referred to as ‘BMC’) layer 231, the RLC layer 241, the MAC layer 271 and the physical layer 291 exist in the U-Plane 202.
The Physical Layer 291 corresponds to Layer 1 of Open System Interworking (OSI) Model 7, and converts data to be transmitted into a radio signal or converts a radio signal into data to be received by performing channel coding/decoding, modulation/demodulation, channelisation/dechannelisation and other functions. The physical layer 291 is connected to the MAC layer 271 by a transport channel 281. The transport channel 281 is classified according to parameters such as a channel coding scheme, a transport block set size transmittable in a unit time and the like. For example, uplink dedicated channels includes a DCH and an E-DCH. The DCH signifies an Uplink Dedicated Channel and the E-DCH signifies an Enhanced Uplink Dedicated Channel.
The MAC layer 271 functions to transmit data, which the RLC layer 241 has transmitted thereto over a logical channel 261, to the physical layer 291 over an appropriate transport channel 281 and transmit data, which the physical layer 291 has transmitted thereto over a transport channel 281, to the RLC layer 241 over an appropriate logical channel 261. The MAC layer 271 may also function to insert additional information into dada transmitted over the logical channel 261 or the transport channel 281, or analyze the inserted additional information to operate appropriately. The MAC layer 271 also controls a random access operation.
The RLC layer 241 takes charge of setup and release of the logical channel 261. Entities 251, 252, 253, 254 of the RLC layer 241 may operate in one operation mode from among an Acknowledged Mode (hereinafter referred to as ‘AM’), an Unacknowledged Mode (hereinafter referred to as ‘UM’) and a Transparent Mode (hereinafter referred to as ‘TM’). There are differences in functions provided according to the respective modes. In general, the RLC layer 241 supervises a function of dividing or assembling a Service Data Unit (SDU), which comes down from an upper layer, into an appropriate size, a function of error correction through an Automatic Repeat Request, and the like.
The PDCP layer 221 is located in a superior position to the RLC layer 241 in the U-Plane, and takes charge of a header compression function of data transmitted in the form of an IP packet, a lossless data transmission function under the condition where the RNC providing services to the UE is changed due to the mobility of the UE, and so forth. The BMC layer 231 is also located in a superior position to the RLC layer 241 in the U-Plane, and supports a broadcasting service in which the same data are transmitted to unspecified plural UEs in a specific cell.
The RRC layer 211 takes charge of assignment and release of radio resources between the UTRAN and the UE. Using RRC connections, the RNC manages the radio resources assigned to the UEs, which are in an RRC connected mode, controls the mobility of the UEs, and transmits CN signals, which must be transmitted to the UEs, to the corresponding UEs. The RNC also manages the RRC connections of the UEs located in the Node B areas under its control.
The E-DCH refers to an uplink dedicated channel which has been developed so as to enhance transmission performance in reverse communication, that is, uplink communication from UEs to Node Bs in the UMTS system. For supporting more stable high-speed transmission, the E-DCH supports various technologies such as Adapted Modulation and Coding (hereinafter referred to as ‘AMC’), Hybrid Automatic Retransmission Request (hereinafter referred to as ‘HARQ’), Node B-controlled scheduling, shorter Transmission Time Interval (hereinafter referred to as ‘TTI’) size and the like.
The AMC is a technology for improving use efficiency of resources by determining modulation and coding schemes dependent on channel conditions between a Node B and a UE. A combination of modulation and coding schemes is called a Modulation and Coding Scheme (hereinafter referred to as ‘MCS’). It is possible to define various MCS levels according to supportable modulation and coding schemes. The AMC adaptively determines the levels of the MCS dependent on the channel conditions between the Node B and the UE to improve the use efficiency of resources.
The HARQ signifies a technology for retransmitting data packets so as to compensate errors occurring in initially transmitted data packets. The HARQ may be classified into a Chase Combining (hereinafter referred to as ‘CC’) technique for retransmitting data packets having the same formats as those of the initially transmitted data packets when the errors occur and an Incremental Redundancy (hereinafter referred to as ‘IR’) technique for retransmitting data packets having different formats than those of the initially transmitted data packets when the errors occur.
The Node B-controlled scheduling signifies a transmission scheme in which if the Node B determines whether or not uplink data is transmitted, an upper limit value of possible data rates and so forth to transmit the determined information as a scheduling command to the UE when the data is transmitted using the E-DCH, the UE determines a possible data transmission rate of the uplink E-DCH with reference to the scheduling command to transmit the data at the determined data transmission rate.
The shorter TTI size permits a TTI of 2 ms shorter than the minimal TTI of the existing GSM/GPRS system, that is, 10 ms, thereby reducing retransmission delay time and thus enabling high system throughput.
FIG. 3 is a view for explaining uplink transmission over the E-DCH in a radio link.
Reference numeral 310 designates a base station supporting the E-DCH, that is, a Node B, and reference numerals 301, 302, 303 and 304 designate UEs using the E-DCH. As shown in the drawing, the UEs 301 to 304 individually transmit data to the Node B 310 over the E-DCH. The Node B 310 utilizes data buffer statuses, requested data transmission rates, or channel condition information of the UEs 301 to 304 using the E-DCH, to inform the possibility of data transmission over the E-DCH, or to perform a scheduling operation for adjusting an E-DCH data transmission rate UE by UE.
In order to improve the overall system performance, the scheduling is performed in such a manner that lower data transmission rates are assigned to the UEs 303, 304 remote from the Node B 310 and higher data transmission rates are assigned to the UEs 301, 302 adjacent to the Node B 310 while a Noise Rise or Rise over Thermal (ROT) value dose not exceeds a target value.
The E-DCH may be supported by Node Bs supporting 3GPP (3rd Generation partnership Project) Release Apr. 5, 1999 and cannot be supported by the existing Node Bs, for example, Node Bs supporting only 3GPP Release 6. Therefore, in an environment where all of cells cannot support enhanced 3GPP standards, for example, in a network which includes cells supporting the E-DCH and cells not supporting the E-DCH, a new technology is desired to enables a UE moving among the cells to selectively use the E-DCH or the DCH as a transmission path for uplink data transmission.